1. Field of the Invention
The present invention relates to a fuel cell system, particularly to a fuel cell system to recover the high temperature gas from the after burner to the reformer for increasing the heat efficiency and the method of heat recovery thereof.
2. Description of the Prior Art
Because the energy price soars continuously at present, thus, how to solve the energy problem is become the attention focus of every country, wherein the best way is to raise the energy efficiency.
The fuel cell is an apparatus to convert the chemical energy into the electric energy through the electrochemical reaction. Its working principle is to transport the hydrogen-containing fuel and the oxide (air or oxygen) to the anode and the cathode of cell, separately. The anode decomposes the fuel into the hydrogen ion and the electron. The hydrogen ion passes through the proton exchange membrane from the anode to the cathode, which reacts with the electron transmitted from the outside circuit into water at the cathode. If the fuel is supplied continuously, the fuel cell will be able to generate the power. Because the reaction product of fuel cell is water, thus there is no environmental pollution. The development of this technology has been paid widely attention due to its high efficiency and low pollution.
Please referring to FIG. 1, which shows a structure plot of the conventional fuel cell system 100. The preferred one is the solid oxide fuel cell combined heat and power (SOFC-CHP), but it is not limited to this. The abovementioned system 100 comprises a solid oxide fuel cell 10 to generate the electric energy by chemical reaction, a reformer 11 to reform the fuel to hydrogen rich gas, a first heat exchanging unit 12, a second heat exchanging unit 13 and an after burner 14. In addition, the first heat exchanging unit 12 comprises a methane input part 12a, a methane output part 12b, an exhaust gas input part 12d and an exhaust gas output part 12c connected with the after burner 14. The second heat exchanging unit 13 comprises an exhaust gas input part 13a connected with the exhaust gas output part 12c of the first heat exchanging unit 12, an exhaust gas output part 13b, an air input part 13c and an air output part 13d. The reformer 11 comprises a methane input part 11a. connected with the methane output part 12b of the first heat exchanging unit, a hydrogen rich gas output part 11b connected with the fuel cell and a water input part 11c. 
Basically, the electrochemical reaction of fuel cell system 100 is an exothermic reaction. The reforming reaction for generating the hydrogen from methane is an endothermic reaction. Therefore, the main function of the after burner 14 is to convert the unreacted gas and fuel in fuel cell 10 and reformer 11 into heat energy and output it to the first heat exchanging unit 12. The first heat exchanging unit 12 and the second heat exchanging unit 13 can further use the recovered heat energy to orderly heat methane and air. The heated methane is able to assure the reforming reaction at suitable temperature, in order to achieve sufficient efficacy of raising efficiency and conserving energy.
However, there are some limitations in applying the conventional scheme, such as:
(1) The working temperature of reformer 11 is between 400° C. and 700° C. If there is no external energy for the reforming reaction, the energy required by endothermic reaction should be provided by methane and water vapor completely. Thus, it is estimated that the exit gas temperature of after burner 14 should be as high as 1750° C. to assure the reforming reaction in the range of its working temperature. However, the high-temperature heat exchanging unit will be required for the high-temperature gas, which will be unfavorable to the operation life and price of system.
(2) The gas temperature for the anode and gas temperature for the cathode of fuel cell 10 should be close (temperature difference should be less than 150° C.), otherwise the cell pile will break because of heat stress. However, in the abovementioned scheme, because the heat exchanging mechanism is too simple, it is necessary to use accurate systematic parameter and design to close the abovementioned temperature.
(3) In the abovementioned scheme, only a little part of recovered waste heat that used for heating the methane and air, most waste heat is used for other uses, such as providing the hot water or hot gas. However, the demand of hot water/hot gas is not high in the subtropical area, thus the abovementioned scheme is not suitable for the subtropical area.